<!DOCTYPE html>
<html>
<head>
    <title>Homework</title>
    <!-- 外部样式 -->
    <link rel="stylesheet" href="stylefile1.css">
</head>
<body>
    <header>
        <p>A</p>
    </header>
    <section>
        <div class="left-column">
            <p>B</p>
        </div>
        <div class="right-column">
            <p>C</p>
        </div>
    </section>
    <footer>
        <p>D</p>
    </footer>
</body>
</html>